Facing mail delivery issue.

Hi @Noyon Chandra Das

Thank you for posting your question in Microsoft Q&A. 

From my perspective, this issue might be a temporary disruption related to DAG replication, Active Directory, or the Transport Service, causing the Public Folder database status to show as Unknown and the queue to enter a Retry state. Once the system stabilizes, the status typically returns to Mounted, and the queue is automatically released. 

This could be due to several possible causes: 

1. Replication/Communication Issue within DAG:

Since Exchange operates in a DAG, there may have been a temporary network or replication issue between nodes.  If the Public Folder database copy on the active node becomes disconnected or enters a ServiceDown state, Exchange may report the database as Unknown, and related queues may become congested. 

2. Resource/Performance Bottlenecks:

If the server experiences a temporary shortage of resources (CPU, memory, or I/O), database mounting or replication may slow down, causing the Exchange Admin Center (EAC) to report the database as Unknown.  Once resources stabilize, the database status typically returns to Mounted, and the queue is cleared. 

3. Transport Service Disruption 

If the Microsoft Exchange Transport Service or Microsoft Exchange Replication Service is overloaded or temporarily restarted, it can cause the Public Folder queue to enter a Retry state. 

4. Temporary AD or Cluster Issues 

Exchange relies heavily on Active Directory and cluster quorum. A brief interruption in AD replication or cluster witness availability may cause the database status to show as Unknown, which usually resolves itself once synchronization resumes. 

You could try to use this command to get more detail: 

Get-Queue | Format-List 

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